THE FAR-ULTRAVIOLET SPECTRUM OF NGC-4151 AS OBSERVED WITH THE HOPKINS-ULTRAVIOLET-TELESCOPE ON ASTRO-2

We observed NGC 4151 on six separate occasions at intervals of 1 to 3 days using the Hopkins Ultraviolet Telescope during the flight of Astr o-2 aboard the space shuttle Endeavour in 1995 March. The far-UV spect ra cover the spectral range from the interstellar cutoff at 912 to 184 0 Angstrom with a resolution of 2-4 Angstrom. The mean spectrum, repre senting 4752 s of integration time, has a signal-to-noise ratio of app roximately 30, and it shows profound differences compared to that obta ined during the flight of Astro-l in 1990 December. The continuum, 5.3 x 10(-13) erg cm(-2) s(-1) Angstrom(-1) at 1455 Angstrom, is 5 times brighter, the brightest UV flux ever observed for NGC 4151. All high-i onization absorption lines have strengthened considerably-S VI lambda lambda 933, 945, C III lambda 977, O VI lambda lambda 1032, 1038, N V lambda lambda 1239, 1243, Si IV lambda lambda 1394, 1403, and C IV lam bda lambda 1548, 1551. The Lyman series absorption lines have also inc reased in strength, with the bulk of the absorption requiring a neutra l hydrogen column density of 5 x 10(17) cm(-2) covering only 78% of th e UV source with an effective Doppler parameter of 350 km s(-1). Howev er, up to 5 x 10(20) cm(-2) of neutral hydrogen that fully covers the source could be present in gas with a thermal Doppler parameter of 20 km s(-1). Single-zone photoionization models of warm absorbing gas are unable to account for both the X-ray absorbing material and the UV-ab sorbing gas, largely because of the wide range of ionization states pr esent in the UV and the X-ray.